In a bowl, combine miso paste, cooking sake, mirin, brown sugar, and soy sauce
Whisk until combined
Place chicken thighs in a zip top bag and pour mixture over chicken, reserving about 1/4 C for basting
Marinate for at least 30 minutes, with 4+ hours being preferred (up to overnight)
Heat oil in a sturdy pan over medium-high heat until the oil is shimmering
Remove chicken from marinade and place skin side down in the pan (discard marinade*)
Sear for 5-7 minutes depending on size. Attempt to flip. If the skin does not easily release from the pan, continue cooking until the skin releases.
Cook for another 5-7 minutes on the other side
Cover and remove from heat allowing the temperature to rise to 160℉ when a meat thermometer is inserted into the thickest part of the thigh
Once temperature reaches 160℉, baste each thigh with the reserved mixture. Re-cover and let rest until temperature reached 165℉.